Within the last ten years there has been a substantial movement away from the use of older traditional explosive compositions such as TNT, dynamite, and nitroglycerin as blasting agents for hard rock mining, excavation, and similar commercial purposes, in favor of a readily available prilled ammonium nitrate (AN) or mixtures of ammonium nitrate with other similar oxidizer salts, plus various organic and inorganic fuels.
Such explosive formulations use relatively cheap components, can be produced or constructed "in situ", and are much safer compared with traditional explosives since no long term storage or transportation is generally required.
Where an intended bore hole is deep and/or wet, however, AN and ANFO formulations are easily desensitized and must be protected (a) by special packaging, (b) by the addition of expensive AN coatings, and (c) by thickeners or gelling agents, and the like. Moreover, ANFO has somewhat limited blasting energy due to its low bulk density.
Some of the above problems can be avoided, in part, by use of special slurries having a higher bulk density, but such formulations are usually not moisture proof, and still must rely upon gassification or other means to retain an acceptable level of sensitivity. Pressure conditions at the bottom of deep bore holes, however, may compress gas voids, which can result in temporary non-detonatable conditions, leaving partial or residual unexploded charges in areas where further drilling, mining or excavation may be required.
A substantial breakthrough with respect to moisture resistance is described in U.S. Pat. No. 3,161,551 of Egly, in which a water-resistant blasting agent is obtained having at least one solid prilled inorganic salt (e.g. ammonium nitrate), treated with a 50-70% AN solution in the form of an aqueous invert emulsion containing a fuel oil and a long chain fatty acid or derivative thereof as the continuous hydrophobic phase.
Egly's composition exhibits substantial resistance to water because of the fact that the water-in-oil emulsion fills most of the natural voids in the solid prilled AN salt component, and water cannot easily force its way through the continuous external hydrophobic phase of the emulsion. Sensitivity problems remain, however, when deep-hole blasting is desired.
Bluhm (U.S. Pat. No. 3,447,978) also describes a class of explosive slurry compositions generally falling within the category of water-in-oil emulsion blasting agents, what consist of a discontinuous AN aqueous phase (optionally supplemented by other water soluble oxidizer salts) in a continuous phase consisting of a carbonaceous fuel having a predetermined gas-retaining consistency at 70.degree. F., plus voids, inclusive of an occluded gas component such as air and glass bubbles, plus a standard water-in-oil emulsifier.
Bluhm's compositions, while capable of avoiding some of the above-enumerated deep wet-hole problems, are often difficult to prepare at the hole without substantial agitation, despite the use of art-recognized water-in-oil type emulsifiers.
Emulsifying agents found usable in Egly and Bluhm include fatty acid derivatives as listed, for instance, in U.S. Pat. Nos. 3,161,551, 3,447,978, 3,765,964, 4,110,134 and UK No. 1,306,546. Based on such background disclosure, it may be generally assumed that sorbitan fatty esters such as sorbitan oleates are preferred in the art for obtaining stable invert emulsions but at considerable sacrifice, timewise. For comparison purposes, however, emulsifiers can also include faster acting nitrogen-containing surfactants such as ammonium salts as exemplified in U.S. Pat. Nos. 4,026,738 and 4,141,767, unsaturated fatty bis(hydroxyethyl) oxazolines (e.g. U.S. Pat. Nos. 4,216,040, 4,322,258), fatty 1-hydroxyethyl-2-imidazolines (e.g. U.S. Pat. No. 4,315,784) and sodium N-methyl-N-alkyl (beef tallow) Taurate (e.g. U.S. Pat. No. 4,315,787). The increased efficiency of the N-containing compounds, however, is usually more than offset by loss in emulsion stability.
Due to the above-indicated slowness of the fatty acid esters and the concurrent real need for pressure and water-resistant properties, it has frequently become necessary to resort to lengthy and severe agitation to form a stable invert emulsion. This, in turn, however limits options such as use of glass bubbles, sintered microspheres or similar density control components due to both physical and chemical instability of such formulations when exposed to substantial shearing forces.
It is an object of the present invention to increase the utility of invert emulsion blasting agents, particularly emulsions utilizing glass bubbles or similar void-maintaining additive material as density control agents, when produced, in situ, using simple equipment.
It is a further object of the present invention to more easily form stable water-in-oil emulsion blasting agents, using a limited amount of agitation.